If you want to export the list of known issues to Excel, you can only export the first 100.  This is a setting done by MS.  You can change this on your TFS server.  Go to "<Disk>:\Program Files\Microsoft Visual Studio 2008 Team System Web Access\Web" and open here the web.config file.  Look for the setting "<maxWorkitemsInReportList count="100"/>".  here you cange the value 100 to another value.

But one remark.  If you set this number too high, you will encounter some performance issues.